1. Field of the Invention
The present invention relates to the etching of dielectric layers in miniature multilayer devices, such as semiconductor integrated circuits (ICs), and particularly to endpoint detection of low open area dielectric etch processes.
2. Prior Art
The manufacture of miniature devices such as ICs and other micro-electronic devices formed on a substrate uses etching of dielectric material during many of the manufacturing process steps. An example of one very important step is selective plasma etching of dielectric layers. Typically, the dielectric is silicon dioxide and it is used as an insulating layer in the device structure. In order to complete the IC structure, many layers must be deposited and etched sequentially to integrate the semi-conducting, conducting and insulating layers which comprise the IC. Among the dielectric etch processes, formation of the device contacts, called xe2x80x9ccontact etchxe2x80x9d, and formation of the vias by which the metal conducting lines are connected, called xe2x80x9cvia etchxe2x80x9d, are critical to the device functionality and integrity.
An important aspect of all semiconductor plasma etch processes is determining when sufficient material has been etched and stopping the process prior to inadvertently removing layers below and destroying the device. For example, during contact etch, it is essential that the etch process be terminated at the silicon contact. Over-etching would remove portions of the active silicon, even if the oxide etch chemistry is highly selective to oxide over silicon, and would be detrimental to the IC operation. Similarly, via etching must stop on top of the underlying metal layer to maintain device integrity. Over-etching is also costly as the over-etch can be a significant part of the process time leading to wasteful consumption of gases and machine time.
IC geometries are continually decreasing in size to facilitate higher processing speeds and greater memory capacity of the devices. This means that contact and via holes sizes are getting smaller relative to the area of the wafer as device packing densities increase. The area of exposed oxide to be etched over the total substrate area is known in the art as the xe2x80x9copen-areaxe2x80x9d. As device geometries shrink, the open-area of these etch processes is decreasing, resulting in increased difficulties in determining etch process endpoint.
The sensitivity of any endpoint technique can be defined as the change in measured signal divided by the variance of that signal during the plasma process. It is known in the art as the signal-to-noise ratio for the endpoint signal. This ratio determines the ultimate effectiveness of any endpoint detector. As open areas decrease, it is found that this signal-to-noise ratio also decreases, since the chemical or physical event which defines endpoint occurs over a much smaller area ratio. There is a need therefore for an endpoint system which facilitates endpoint detection in dielectric etch processes that can be extended to very low open areas.
The plasma etch process usually involves placing a silicon wafer substrate coated with various conducting and insulating layers in a process chamber, introducing process gases and applying radio-frequency power to create the plasma. The plasma consists then of ions, electrons, radical gas species and neutral gas, all of which permit the desired reaction to proceed.
The current state of the art relies on the endpoint of the process to be determined from a change in the process gas composition due to breakthrough into the layer below the layer being etched. The uncovered layer normally contains a different mixture of chemical compounds than the etched layer. The presence of chemicals released from the uncovered layer in the plasma process gas can be determined by various methods, based on a change in the impedance of the plasma, the optical light output of the plasma, or position of the match unit: see, for example, U.S. Pat. Nos. 4,207,137; 4,493,745; 4,615,761 and 4,954,212. However, as the open area is reduced and in certain types of etch process such as oxide etch, very small amounts of chemicals are released, and the changes to the plasma are no longer easily detected.
A typical commercial endpoint detector in plasma etch systems is based on optical emission from the plasma. The optical sensor is generally set up to monitor light at a particular frequency, or frequencies, associated with etching a specific layer. For example, when etching silicon dioxide through to metal, such as in via etch applications, the optical sensor system may be set up to monitor emissions from both the oxide material and the metal as both are excited by the plasma species. Observation in how these emissions change as the oxide is removed and the metal is exposed facilitates the technique. There is substantial prior art on this technique and recent advances include multiple wavelength observations with signal processing to enhance signal-to-noise as far as possible. However, the ultimate sensitivity of the technique remains limited by the fact that as open-area decreases, the intensity of optical emissions for given species decreases (proportionally and linearly) and can therefore be so low as to be lost in the noise of the system.
There is a need therefore, for a method which overcomes or mitigates the limitations of present endpoint detection schemes, especially on low open area dielectric etch processes.
According to the present invention there is provided a method of manufacturing a miniature multilayer device in which a low open area dielectric layer is selectively etched through to an underlying conductive region using an electrically conducting medium, wherein the endpoint of the etch process is determined by detecting the abrupt change in capacitance across the device just as the final portion of the dielectric layer is removed.
Preferably the electrically conducting medium is a plasma.